The present invention relates to fiber-optic communications networks, and more particularly, to multiwavelength pump systems for Raman amplifiers in fiber-optic communications networks.
Fiber-optic networks are used to support voice and data communications. In optical networks that use wavelength division multiplexing, multiple wavelengths of light are used to support multiple communications channels on a single fiber.
Optical amplifiers are used in fiber-optic networks to amplify optical signals. For example, optical amplifiers may be used to amplify optical data signals that have been subject to attenuation over fiber-optic links. A typical amplifier may include erbium-doped fiber coils that are pumped with diode lasers. Raman amplifiers have also been investigated. Discrete Raman amplifiers may use coils of fiber to provide Raman gain. Distributed Raman amplifiers provide gain in the transmission fiber spans that are used to carry optical data signals between network nodes.
The fiber in Raman amplifiers may be pumped by single-wavelength sources. However, the Raman gain spectrum produced by a single-wavelength source often does not have the spectral shape that is desired.
Amplifier systems with non-flat gain spectra amplify optical signals on channels at different wavelengths by different amounts. This is often not acceptable, particularly in communications links with a number of cascaded amplifiers. Moreover, other non-flat spectral shapes may be desired.
The gain spectrum of a Raman amplifier may be modified using a spectral filter. For example, a gain equalization filter may be used to produce a relatively flat gain spectrum by introducing optical losses that compensate for the non-flat shape of the Raman gain spectrum. However, the optical losses associated with using the filter consume optical power and tend to increase the noise figure of the Raman amplifier.
Another approach for pumping Raman amplifiers involves using a Raman pump source based on multiple diode laser pumps, each of which operates at a different pump wavelength. With this type of approach, the diode laser pumps are each driven at an appropriate current to provide a Raman gain contribution. The overall gain of the Raman amplifier is determined by the Raman gain contributions of each of the individual Raman pump lasers.
If a sufficient number of diode laser pumps are used, the overall gain of the Raman amplifier may be made flat. Because gain equalization filters are avoided, the noise figure of the Raman amplifier may be improved. However, coupling the pump light from each of the individual diode lasers into a single fiber for use in a Raman amplifier may be complex, bulky, and costly.
It is therefore an object of the present invention to provide Raman pumps that produce multiple Raman pump wavelengths.
This and other objects of the invention are accomplished in accordance with the present invention by providing multiwavelength light sources that may be used as Raman pumps for Raman amplifiers. The Raman amplifiers based on the multiwavelength pumps may be used in fiber-optic communications networks having communications links that support channels operating at one or more different wavelengths. The Raman amplifiers may be based on distributed or discrete Raman amplifier arrangements. Raman gain may be provided by pumping fiber with the multiwavelength Raman pump. The fiber may include one or more coils of fiber such as dispersion-compensating fiber, may be a span of transmission fiber, or may be any suitable combination of coils and transmission fiber spans.
The gain spectrum produced by pumping the fiber in a Raman amplifier with the multiwavelength Raman pump may be flat or may have another desired spectral shape. The Raman amplifier may have a control unit. The control unit may be used to control the operation of the Raman pump. For example, the control unit may be used to adjust the pump power produced at each of the pump wavelengths to produce the desired spectral shape for the Raman gain. The control unit may be used to adjust the pump power produced at each of the pump wavelengths to produce the desired spectral Raman gain shape for different types of gain fibers.
Optical monitoring equipment may be used to measure optical signals on the fiber-optic communications link. The optical signal measurements may be used by the control unit in adjusting the pump powers produced by the multiwavelength Raman pump. Monitoring equipment may also be used to measure the pump powers at each of the pump wavelengths. The optical monitoring equipment may be integrated with the pump module.
A power amplifier stage may be used to increase the optical power from the Raman pump that is used to pump the fiber in the Raman amplifier. The power amplifier stage may be an external semiconductor optical amplifier or a fiber amplifier or may be a semiconductor optical amplifier stage that is integrated with the Raman pump.
The multiwavelength Raman pump may be based on a semiconductor device. The device may have multiple waveguide gain sections with different distributed feedback gratings for providing optical feedback at different pump wavelengths. Light from each of the waveguide gain sections may be combined using a multiplexer such as a wavelength multiplexer or a simple Y-junction coupler on the semiconductor device.
An optical coupler such as a lens and isolator arrangement or an integral fiber lens may be used to couple pump light from the multiwavelength Raman pump into a single fiber. The single fiber may be coupled to a fiber that is to be Raman-pumped to produce Raman gain using a pump coupler.
The semiconductor device may be mounted on a heat sink. A temperature sensor may be used to monitor the heat sink temperature. A thermoelectric cooling element may be used to maintain the heat sink and semiconductor device at a desired temperature.
A depolarizer may be used to depolarize the Raman pump light provided by the Raman pump. This allows unpolarized Raman gain to be produced in the Raman-pumped fiber.
Gratings may be used to define the lasing wavelengths for the Raman pump. The gratings may be chirped. A relatively small number of grating periods may be used in a grating to broaden the pump linewidth.
A number of tunable sources may be used on a single semiconductor device to provide the multiple pump wavelengths. Tunable sources may be based on waveguide gain sections that have multiple grating regions, each of which has a grating with a different periodicity. Tunable sources may also be provided that use multiple waveguide layers. The wavelength of light produced by such a layered structure may be selected by adjusting the drive current through a grating region on the structure.
The active region on the semiconductor Raman pump device may be provided using multiple quantum wells. The gain spectra of the multiple quantum wells may be configured to provide gain peaks in the vicinity of each of the pump wavelengths.
The active region on the semiconductor Raman pump device may be provided using multiple quantum wires or quantum dots. The gain spectra of the multiple quantum wires or quantum dots may be configured to provide gain peaks in the vicinity of each of the pump wavelengths.
A fiber Bragg grating may be used to form an external cavity or coupled cavity arrangement for the semiconductor device. The fiber may have multiple gratings, each of which corresponds to a different pump wavelength.
Further features of the invention and its nature and various advantages will be more apparent from the accompanying drawings and the following detailed description of the preferred embodiments.